Method of anodizing aluminum



United States Patent 3,276,980 METHOD OF ANODIZING ALUMINUM Robert P. Basco, Roseville, and Lawrence V. Puls, Madison Heights, Mich, assignors to General Motors Corporation, Detroit, Mich., a corporation of Delaware No Drawing. Filed Jan. 29, 1963, Ser. No. 254,569 2 Claims. (Cl. 204-58) This invention relates to anodic surface treatment of aluminum and aluminum alloys and more particularly to a treatment directed to the improvement of the reflectivity of such surfaces.

it is the conventional practice in finishing aluminum surfaces for decorative purposes to first clean and polish the aluminum surface by suitable means and thereafter anodizing the polished surface. The polishing step is generally accomplished by mechanical buffing or by immersing the aluminum in a suitable hot etchant bath. In some instances the chemical treatment may be followed by mechanical buffing. The polished aluminum surface is usually anodized to preserve the reflectivity resulting from the polishing step. However, the anodizing step tends to dull to a considerable extent the reflectivity produced by the polishing step.

It is the basic object of this invention to provide an anodizing process adapted for use after the aluminum polishing procedure whereby the dulling effect of the anodizing step is substantially eliminated and in some instances the reflectivity is actually improved. In general, this and other objects of the invention are accomplished by anodizing the polished aluminum in a sulfuric acid-type electrolyte containing a small proportion of a compound taken from the group consisting of ammonium tartrate, boric acid and ammonium borate, and preferably containing ammonium tartrate. Other objects and advantages of the invention will be apparent from the following detailed description. d In general, the method of this invention as indicated above involves the basic concepts of polishing or buffing the aluminum surface to achieve a maximum or desired brightness thereof and subsequently anodizing the brightened surface to provide it with permanence. A wide variety of chemical processes have been developed for bright polishing the aluminum surfaces. One of the more effective and popular processes in commercial use involves dipping the aluminum for a few seconds into an acid bath heated to about 200 F. consisting essentially of about 80% phosphoric acid, about 4.2% nitric acid and the balance water and small amounts of copper nitrate and other substances adapted to improve the effectiveness of the acid bath. Alternatively, the aluminum may be polished by a mechanical buffing procedure well known in the art. A combination of chemical and mechanical polishing may also be used.

A specific illustration of the invention is as follows. A panel of AA5457 aluminum is first polished and buffed. Next, the panel is degreased by means of trichloroethylene vapors. The panel is then soaked in an alkaline cleaning solution for about 3 minutes at 150 F. Following this, the panel is rinsed in cold distilled water and is now ready for anodizing. An alternate polishing process may include the steps of degreasing the panel with trichloroethylene vapors, soaking the panel in alkaline solution and then dipping the panel in an acid bath consisting of 80% by weight of phosphoric acid and 4.2% by weight of nitric acid for about 45 seconds at a temperature of 195 F. to 200 F. The alternate process may be completed by rinsing the panel with water and permitting it to air dry.

Next, the panel is anodized in an aqeous acid solution consisting of about 15% sulfuric acid and about 2.0%

3,276,980 Patented Oct. 4, 1966 ammonium tartrate. The anodizing procedure is carried out at a temperature of about 78 F. with a current density of about 12 amperes per square foot.

Finally, the anodized coating is sealed by dipping the anodized panel in an aqeous solution containing about 0.1% of nickel acetate which has been adjusted to a pH of about 5.8 with acetic acid. By measuring the reflectivity before and after the anodizing step, it is found that the haze is reduced.

The above procedure was repeated with .a number of panels involving varied amounts of ammonium tartrate, boric acid and ammonium tetraborate under different temperature conditions and different concentrations of sulfuric acid which are set forth in the table.

It will be noted from this table that in instances where a conventional sulfuric acid bath is used, there is a marked increase in the haze following the anodizing procedure as may be seen in comparing columns 7 and 8 for the panels 1 through 3. In instances where ammonium tartrate has been added, no appreciable increase in haze is observed (panels 7 through 12) and in some instances, particularly in instances involving relatively small amounts of ammonium tartrate in the neighborhood of 1% to 3%, the haze is actually reduced (panels 3 through 6). The range of values in columns 7 and 8 of the table result from about five readings taken randomly on each panel.

As indicated in the table, test panels 13 through 15 were polished and anodized using an anodizing electrolyte containing 1% to 4% ammonium borate. Reflectivity tests indicated only a small increase in haze, far less than the additional haze produced in panels 1 through 3 involving a standard sulfuric acid electrolyte. Similarly, panels 16 and 17 were polished and anodized using a sulfuric acid solution containing 0.6% to 4% boric acid. Reflectivity measurements again indicated a negligible or small increase in haze.

The haze in the above experiments was measured as a percent of light reflected from the aluminum surface involved. A Bausch and Lomb Spectronic 20 Colorimeter adjusted for a wave length of 535 millimicrons was used for making these measurements.

The anodizing process of this invention has been found effective in using sulfuric acid anodizing baths containing from about 10% to 30% sulfuric acid operated at a minimum temperature of about 65 F. and a maximum temperature of about 115 F. Below 65 F. an undesirable hard dark type coat is produced whereas above 115 F. the aluminum is etched rather than anodized. Current densities of from about 4 to amperes per square foot may be employed without great differences in the quality of the anodized coating. Preferably, sulfuric acid concentrations of about 12% to 17% are employed. It has been found that ammonium tartrate concentrations in the vicinity of 0.01% to 0.02% produce brightness improvement. Superior results are obtained using at least about 1% ammonium tartrate. Ammonium tartrate concentrations of about 2% to 3% have been found optimum. However, ammonium tartrate concentrations may range up to the saturation point in the vicinity of 12% to 15% with satisfactory results. It is preferred to use sulfuric acid concentrations of about 17.5% operated at a temperature of about 83 F. Markedly beneficial results are obtained in electrolyte solutions containing boric acid or ammonium borate in the range of about 1% to 4%, about the saturation concentrations of these salts in the electrolyte. Similar results have been obtained using other aluminum material such as AA5557 and SXXX. The term aluminum when used in the appended claims is intended to include both aluminum and aluminum alloys in which the aluminum U constituent predominates and which are typically employed in brightening and anodizing procedures.

While the embodiments of this invention as disclosed herein constitute preferred forms, it is to be understood that other forms may be adopted without departing from the spirit of the invention.

4 current anodic treatment in an aqueous electrolyte consisting essentially of about 10% to 30% sulfuric acid and ammonium tartrate in proportions of from 1% to 4% by weight at a temperature of from about 65% F. to 115 F. and at a current density of about 4 to 100 amperes H2804 Ammonium Ammonium Panel No. Temp. Cone. Tartrate Borate F.) (Percent (Percent (Percent Wt. Wt.)

5 per square foot.

Boric Acid Butfiing (Percent aze Final Haze (Percent) (Percent) 13. 1-13. 8 18. 2-..1. 16. -17. 6 23. 8-26. 0 28-29 39-41 27-28 -21 27-29 -27 27-29 27-28 25-30 20-35 13-16 15-16 16. 9-17. 5 16. 8-19. 0 16. 0-16. 8 14. 0-18. 5 14. 0-16. 1 14. 9-19. 5 16. 3-19. 2 15. 5-19. 0 14. 9-16. 7 15. 0-19. 7 14. 1-15. 1 15. 9-17. 2 14. 2-15. 9 15. 1-18. 3 4. 0 17. 1-19. 6 19. 3--1. 0 (saturated) N oTE.-Panel 17 was anodized for over 10C0 ampere-minutes per square foot with a resultant nnodic coating coating thickness of over 0.0012 inch.

We claim:

1. In a process for improving the reflectivity of aluminum surfaces including the step of brightening the aluminum surface by subjecting it to a suitable mechanical, chemical, or electrochemical treatment, the step comprising following the said brightening treatment by the step of subjecting the brightened aluminum surface to direct current anodic treatment in an aqueous electrolyte consisting essentially of about 10% to 30% sulfuric acid and ammonium tartrate in proportions of from about 1% to the saturation point thereof at a temperature of from about 65 F. to 115 F. and at a current density of about 4 to 100 amperes per square foot.

2. In a process for improving the reflectivity of aluminum surfaces including the step of brightening the aluminum surface by subjecting it to a suitable mechanical, chemical or electrochemical treatment, the step comprising following the said brightening treatment by the step of subjecting the brightened aluminum surface to direct References Cited by the Examiner UNITED STATES PATENTS OTHER REFERENCES Mason, Ralph B., et al., Anodic Behavior of Aluminum and Its Alloys in Sulfuric Acid Electrolytes, Journal of the Electrochemical Society, February 1954, p. 58.

JOHN H. MACK, Primary Examiner.

HOWARD S. WILLIAMS, Examiner.

G. KAPLAN, Assistant Examiner. 

1. IN A PROCESS FOR IMPROVING THE REFLECTIVITY OF ALUMINUM SURFACES INCLUDING THE STEP OF BRIGHTENING THE ALUMINUM SURFACE BY SUBJECTING IT TO A SUITABLE MECHANICAL, CHEMICAL, OR ELECTROCHEMICAL TREATMENT, THE STEP COMPRISING FOLLOWING THE SAID BRIGHTENING TREATMENT BY THE STEP OF SUBJECTING THE BRIGHTENED ALUMINUM SURFACE TO DIRECT CURRENT ANODIC TREATMENT IN AN AQUEOUS ELECTROLYTE CONSISTING ESSENTIALLY OF ABOUT 10% TO 30% SULFURIC ACID AND AMMONIUM TARTRATE IN PROPORTIONS OF FROM ABOUT 1% TO THE SATURATION POINT THEREOF AT A TEMPERATURE OF FROM ABOUT 65*F. TO 115*F. AND AT AA CURRENT DENSITY OF ABOUT 4 TOO 100 AMPERES PER SQUARE FOOT. 